The Brightness of the Stars

Apparent Magnitude

The Dome of the Sky shows stars down to magnitude 5.49, although the magnitudes are rounded to whole numbers as in the following scale:

magnitude scale

The intensities of the dimmer stars in particular are represented reasonably accurately. This makes the constellations look reasonably like their appearance in the actual sky. However, it does make it difficult to see the dimmer stars. You will have to look really hard to see a star of magnitude 5 in the Dome sky pictures.

The Magnitude Concept

Hipparchus Classifies the Stars

The concept of measuring the brightness of stars by apparent magnitude can be traced back to the efforts of the Greek astronomer Hipparchus (190 - 120 B.C.E.) to classify the stars into 6 brightness classes.

Hipparchus Used His Eyes

The modern scale of apparent magnitude is in rough agreement with Hipparchus' classification of the brightness of stars. However, Hipparchus could not discriminate well between stars of similar brightness, because he was working with his unaided eye. Hipparchus had no photometric instruments with which to discriminate between similar stars.

Modern Magnitudes

With modern photometry, it is possible to measure fractional magnitudes. The magnitude scale is in fact logarithmic; one step in magnitude corresponds to a factor of 2.512 in actual brightness (that is, in luminous flux).

The Magic Number 2.512

Why this strange factor of 2.512? That number is just the fifth of root of 100. To keep in rough agreement with Hipparchus' brightness classification, five magnitude steps are taken to correspond to a factor of 100 in luminous flux.

The dimmest star that you can expect to see with good eyesight under excellent viewing conditions would be of apparent magnitude 6.5, the "standard limiting magnitude." A star of magnitude 1.5 will be 100 times brighter.

Example Magnitudes

See this site for examples of the magnitudes of various celestial objects.  

Copyright © 1998 - 2010 by Arnold V. Lesikar,
Professor Emeritus
Dept. of Physics, Astronomy, and Engineering Science,
St. Cloud State University,St. Cloud, MN 56301-4498

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